On September 20, 2001, nine days after the attacks that killed 2,977 people in New York, Washington, and Pennsylvania, George W. Bush addressed a joint session of Congress. He declared a war on terror that would not end until every terrorist group of global reach had been found, stopped, and defeated. He warned other nations: “Either you are with us, or you are with the terrorists.” He promised a campaign unlike any other, fought on many fronts, by many means, sustained over years. He did not say it would end. He could not say it would end, because he was declaring war not against a country or an army but against a tactic, and tactics do not surrender.

The War on Terror that Bush declared in September 2001 was neither a war in the traditional sense nor a campaign that could be won in the traditional sense. Over the two decades that followed, it produced two major invasions, dozens of smaller military operations across multiple continents, a drone programme that killed thousands in countries with which the United States was not at formal war, a detention and interrogation system that became a global symbol of democratic governments abandoning their values under pressure, and an intelligence and security apparatus whose scale and intrusiveness had no precedent in American history. It killed approximately 929,000 people by the most comprehensive estimates, including 7,052 American military personnel and hundreds of thousands of civilians in Afghanistan, Iraq, Pakistan, Yemen, Syria, and elsewhere. It cost between four and six trillion dollars by analyses that include veterans’ healthcare and long-term obligations. And after twenty years, the Taliban was back in power in Afghanistan, Al-Qaeda’s affiliates were operating across a wider geography than before, and the Islamic State had emerged, conquered territory, and been militarily defeated but not destroyed.

Understanding the War on Terror requires understanding both what it was designed to accomplish and what it actually produced, holding the genuine achievements, primarily the disruption of Al-Qaeda’s original operational network, alongside the genuine failures, primarily the Iraq War’s consequences and the democracy-promotion agenda’s collapse, in a framework that neither dismisses the threat the policy was designed to address nor pretends the policy successfully addressed it. To trace the arc from the September 11 attacks through Afghanistan, Iraq, the drone campaigns, and the ISIS emergence to the Afghanistan withdrawal of 2021 is to follow the defining foreign policy experience of the early twenty-first century and one of the most consequential sets of strategic decisions in American history.

The Afghanistan Campaign: Operation Enduring Freedom

The October 2001 invasion of Afghanistan, Operation Enduring Freedom, was the most directly justified military action of the War on Terror and the one that initially achieved its stated military objectives with remarkable speed before failing to achieve the political objectives that victory in any sustainable sense required.

The military justification was straightforward: Al-Qaeda had planned and executed the September 11 attacks from Afghanistan under Taliban protection; the Taliban government had refused American ultimatums to surrender bin Laden; and destroying both the Taliban government and Al-Qaeda’s Afghan infrastructure was a legitimate military response to an act of mass murder. The international support was genuine and broad, with the UN Security Council authorising the use of force, NATO invoking Article 5, and dozens of countries either joining the coalition or providing basing, overflight, and intelligence support.

The initial campaign, which combined American and British air power with the Northern Alliance’s ground forces and small numbers of CIA and special operations personnel, achieved a military success that the Taliban’s subsequent resilience has somewhat obscured in memory. Within approximately two months, the Taliban government had collapsed, its forces had been dispersed or had melted into the population, and the major Afghan cities were under the control of the Northern Alliance and the new Afghan Interim Authority that the Bonn Agreement of December 2001 established. The speed of this achievement generated the initial assessment that Afghanistan’s transformation could be managed relatively quickly and at relatively modest cost, an assessment that proved comprehensively wrong.

The failure at Tora Bora in December 2001, when bin Laden escaped through the mountains into Pakistan as American commanders relied on Afghan proxy forces rather than American troops for the direct pursuit, was the campaign’s most consequential single operational failure. The decision to use Afghan allies, taken partly to avoid the appearance of American boots on the ground in a way that might complicate future operations and partly from genuine assessment of the tactical situation, allowed the Al-Qaeda leadership to escape to Pakistan, where they would direct the movement for the following decade.

The Mission Expands: Nation-Building in Afghanistan

The transition from the initial counter-terrorism objective of destroying Al-Qaeda’s Afghan infrastructure to the nation-building mission of creating a stable democratic Afghan government was one of the War on Terror’s most consequential strategic decisions, and it was made largely by incremental default rather than by deliberate policy choice.

The logic was seemingly straightforward: if Afghanistan was left ungoverned after the Taliban’s removal, it would become a sanctuary for terrorism again; therefore, creating a functioning Afghan government was a precondition for sustained counter-terrorism success. This logic was broadly correct as far as it went, but it underestimated the scale of what building a modern state from the foundation of one of the world’s least developed countries actually required, and it was never matched by the resources and attention that the Iraq War’s priority absorbed.

The Afghan government that was built under Presidents Karzai (2001-2014) and Ghani (2014-2021) was characterised by corruption, dependence on American and NATO support, and an inability to extend meaningful governance to most of the country’s territory. The specific dynamic of the Afghan political economy, in which American aid and military contracts created incentives for the political elite to maintain the conflict rather than resolve it, was documented extensively by the Special Inspector General for Afghanistan Reconstruction (SIGAR), whose quarterly reports provided a running account of programmes that were failing while official assessments claimed progress.

The Taliban, meanwhile, regrouped in Pakistan’s tribal areas under ISI protection or tolerance, rebuilt their organisational infrastructure, and began conducting the insurgency that by 2006 had established them as a significant military force in southern and eastern Afghanistan. The assumption that the Taliban had been definitively defeated in 2001 proved wrong; they were dispersed rather than destroyed, and their ability to reconstitute in Pakistan demonstrated that a counter-insurgency in Afghanistan that did not address the Pakistani sanctuary was addressing symptoms rather than causes.

The Iraq War: The Decisive Mistake

The March 2003 invasion of Iraq was the War on Terror’s most consequential and most contested decision, a conflict that had at best a tenuous connection to the September 11 attacks but whose planning and execution defined the War on Terror’s character and whose consequences have continued to shape the Middle East and American foreign policy for more than two decades.

The stated justification for the Iraq invasion rested on two claims: that Saddam Hussein possessed weapons of mass destruction that posed a threat to American and allied security, and that there was a connection between Saddam’s government and Al-Qaeda that made Iraq part of the broader War on Terror. Both claims proved to be wrong. The weapons of mass destruction that American, British, and other intelligence services assessed Saddam possessed were not found after the invasion; the intelligence assessments on which the claims were based were flawed and in some cases fabricated by unreliable human intelligence sources. The connection between Saddam and Al-Qaeda, which the 9/11 Commission found no credible evidence for, was a claim that polling data showed a majority of Americans believed at the time of the invasion, partly because the Bush administration had consistently made both claims in the same context.

The actual motivations for the Iraq invasion are more complex than either the stated justifications or the simple “it was all about oil” critique. The neoconservative intellectual framework that shaped the Bush administration’s foreign policy envisioned a democratic transformation of the Middle East, in which a successful democratic Iraq would demonstrate that Islamic societies could sustain democratic governance and would undermine the authoritarian political culture that the neoconservatives believed generated terrorism. Dick Cheney’s conviction that Saddam represented an intolerable risk that a post-September 11 world could not afford to leave unaddressed, whatever the specific intelligence said, was a political judgment that the emotional aftermath of September 11 made harder to contest than it should have been.

The invasion’s initial military phase was rapid: Baghdad fell on April 9, 2003, forty-one days after the invasion began, and Bush’s May 1 “Mission Accomplished” speech aboard the USS Abraham Lincoln announced the end of major combat operations with a premature confidence that the subsequent years would devastate. The occupation that followed was catastrophically mismanaged. The decisions to disband the Iraqi army, which put approximately 400,000 armed men out of work without income or alternative employment, and to conduct a de-Baathification process that excluded experienced Sunni officials from the new government, were both recommended by the Coalition Provisional Authority under L. Paul Bremer and implemented over military objections that proved entirely prescient.

The Iraqi Insurgency and the Surge

The insurgency that developed in Iraq following the invasion was both predictable and predicted by military and intelligence professionals who had advised against the invasion or against the specific management decisions that catalysed it. By 2004, an insurgency combining former Iraqi military personnel, Baathist loyalists, Sunni tribal leaders who had lost power and status, and an increasingly significant foreign jihadist element under Abu Musab al-Zarqawi had made significant parts of Iraq ungovernable at sustainable cost.

Al-Qaeda in Iraq (AQI), which Zarqawi built from the foreign fighter networks that flowed into Iraq through Syria from 2003, was not present in Iraq before the invasion. Its emergence was a direct consequence of the invasion’s creation of the conditions, ungoverned territory, disaffected Sunni population, available weapons, and the specific propaganda value of fighting Americans in the heart of the Arab world, that Al-Qaeda’s leadership needed to recruit globally. The specific irony, that the Iraq invasion justified partly on the grounds of fighting Al-Qaeda produced the conditions in which Al-Qaeda in Iraq emerged, was not lost on critics but proved politically impossible to acknowledge within the administration until much later.

The sectarian violence that erupted following the February 2006 bombing of the Al-Askari mosque in Samarra, one of the holiest sites in Shia Islam, transformed the insurgency into something approaching civil war. Sunni and Shia militias conducted mass killings of civilians from the other community; Baghdad, which had been ethnically mixed before the invasion, was ethnically cleansed into segregated Sunni and Shia neighbourhoods by 2007. Approximately 100,000 Iraqis were killed in the sectarian violence of 2006-2007 alone.

The Surge of 2007, in which Bush sent approximately 20,000 additional American troops to Iraq and changed the strategy from handing off security to Iraqi forces to directly providing security to the Iraqi population, was the single most effective American military decision of the War on Terror. Combined with the Sunni Awakening, in which Sunni tribal leaders in Anbar Province switched from supporting the insurgency to cooperating with American forces against AQI because of Al-Qaeda’s brutal imposition of its ideology on tribal communities, the Surge dramatically reduced the violence by 2008. General David Petraeus, who commanded the Surge and who had written the Army’s counter-insurgency field manual, was the War on Terror’s most effective senior military commander.

The Surge’s success was real but not permanent. The political reconciliation between Sunni and Shia communities that the military gains were supposed to enable never occurred under Prime Minister Nouri al-Maliki, whose government systematically marginalised Sunni political leaders and reversed the Sunni Awakening’s institutional gains. The conditions that Maliki’s sectarian governance created, a Sunni population in western Iraq that felt excluded, persecuted, and with no political path to redress its grievances, provided the environment in which the Islamic State grew from AQI’s remnants after the American withdrawal.

The Drone Programme

The drone programme, which expanded dramatically under Obama and continued under subsequent administrations, became both one of the War on Terror’s most effective counter-terrorism instruments and one of its most legally and morally contested dimensions.

The case for drones was both military and strategic. Unmanned aerial vehicles could loiter over targets for extended periods, identify and track individuals with precision that no other platform could match, and strike with relatively small warheads that minimised collateral damage compared to other available platforms. They could operate in areas where American ground forces could not be deployed without triggering diplomatic crises or major military commitments, including Pakistan, Yemen, and Somalia. They could kill Al-Qaeda leaders before those leaders could plan and execute attacks that might kill thousands of people.

The case against drones, or more precisely against the programme as it was actually implemented, rested on several dimensions. The “signature strike” policy, under which drones targeted not identified individuals but patterns of behaviour associated with militancy, extended targeting beyond those who were actually engaged in planning attacks to those who resembled people who might be planning attacks, a probabilistic approach to killing that had no clear legal basis. The civilian casualties from drone strikes, which the administration consistently underestimated by using a definition of “militant” that included all military-age males in a strike zone unless they were posthumously proved innocent, generated the anti-American sentiment in affected communities that counter-terrorism strategy was supposed to reduce.

The legal framework for drone strikes outside declared war zones was never fully public, relying on classified opinions about the President’s authority to use lethal force against individuals who posed a threat to the United States regardless of their location. The killing of Anwar al-Awlaki, an American citizen who had become an Al-Qaeda in the Arabian Peninsula (AQAP) propagandist and operational planner, in a drone strike in Yemen in September 2011 raised the most direct challenge to the programme’s legal basis: under what legal authority could the American government execute an American citizen without trial? The Justice Department’s legal opinion justifying the strike was not released publicly for years, and when it was released it was heavily redacted.

Guantánamo Bay and Enhanced Interrogation

The detention facility at Guantánamo Bay, Cuba, which opened in January 2002 and which at its peak held approximately 780 detainees, became the most visible institutional symbol of the War on Terror’s departure from the legal standards that American democracy had historically claimed to embody.

The facility’s creation was premised on the legal argument that it was outside United States jurisdiction, so that normal constitutional protections for detained persons did not apply, and outside the jurisdiction of the Geneva Conventions, which the administration argued did not cover “unlawful enemy combatants.” Both legal arguments were eventually rejected by the Supreme Court in a series of decisions that required the administration to provide some legal process to detainees, but the practical effect was that hundreds of people were held for years without charge or trial while these legal battles proceeded.

The interrogation techniques used at Guantánamo and at the CIA’s network of “black sites” in various countries, which included waterboarding, sleep deprivation, stress positions, and other methods that the Senate Intelligence Committee’s report characterised as torture, were authorised through Justice Department legal opinions that defined torture so narrowly as to exclude almost everything the CIA was doing. The programme’s claimed effectiveness, that it produced intelligence that prevented attacks and led to the capture of terrorist leaders, was disputed by the Senate report’s analysis, which found that the programme produced little intelligence that could not have been obtained through non-coercive means and that CIA briefings to oversight bodies about the programme’s effectiveness were systematically inaccurate.

Abu Ghraib, the Iraqi prison where photographs of American soldiers humiliating, degrading, and physically abusing Iraqi detainees were taken and eventually published in April 2004, was a scandal that connected the specific conduct of individual soldiers to the broader legal and institutional culture of a war on terror in which the normal rules were being suspended. The Abu Ghraib photographs circulated globally, providing Al-Qaeda and its affiliates with their most effective recruiting tool and demonstrating to the populations the War on Terror was supposed to win over that the United States was prepared to abandon the values it claimed to be defending.

The Obama Years: Continuity and Change

Barack Obama’s election in 2008 was partly a response to the Iraq War’s failures and to the Bush administration’s War on Terror approach, and his presidency combined genuine changes in approach with substantial continuity in the underlying framework.

Obama’s most significant changes were the end of the torture programme through executive order, the reduced emphasis on the Iraq War after the 2011 withdrawal, the killing of bin Laden in May 2011, and the increased use of drone strikes as an alternative to large ground force deployments. His most significant continuities were the maintenance of the Guantánamo facility despite his executive order directing its closure, the expansion of the drone programme beyond what the Bush administration had operated, the maintenance of most of the surveillance authorities that the PATRIOT Act and executive orders had established, and the use of military force in Libya, Syria, and other countries under authorities that Congress had neither specifically authorised nor debated.

The bin Laden killing, conducted by Navy SEAL Team Six on May 2, 2011, at bin Laden’s compound in Abbottabad, Pakistan, was the War on Terror’s most strategically significant individual action. It removed the symbolic and operational leader of Al-Qaeda, demonstrated that the decade-long intelligence effort had eventually succeeded, and provided the American public with the specific closure that September 11 had demanded. Obama’s announcement of the killing was one of the most watched presidential addresses in American history.

The Libya intervention of 2011, in which NATO air power helped overthrow Muammar Gaddafi in response to his threatened massacre of civilians in Benghazi, applied the Responsibility to Protect doctrine that the Rwandan Genocide had helped generate. Its aftermath, in which Libya descended into civil war among competing armed factions, weapons flooded neighbouring countries, and the Islamic State established a presence in Libyan territory, provided the most direct evidence that military intervention without adequate post-conflict planning could convert a humanitarian operation into a strategic failure.

The Syria policy, or lack thereof, was the Obama administration’s most painful War on Terror failure. The Syrian civil war that began in 2011 and that eventually killed approximately 500,000 people, displaced half the Syrian population, and created the conditions for the Islamic State’s rise in eastern Syria, was addressed by the administration through a combination of half-measures, including a limited programme to train and equip Syrian rebels, that was insufficient to affect the conflict’s outcome while large enough to implicate the United States in its continuation.

The Rise of ISIS

The Islamic State of Iraq and Syria (ISIS, also known as ISIL or Daesh), which declared a caliphate in June 2014 and at its territorial peak controlled approximately 34,000 square miles in Iraq and Syria, was the War on Terror’s most significant strategic failure and the most direct consequence of the Iraq War’s creation of the conditions that Al-Qaeda in Iraq required.

The organisation grew from Al-Qaeda in Iraq, which had been significantly degraded by the Surge but not destroyed. Under Abu Bakr al-Baghdadi’s leadership from 2010, it rebuilt in the sectarian political environment that Maliki’s government had created, recruiting from the pool of Sunni men who had been released from American detention facilities where they had been radicalised, from the former Iraqi military officers whose exclusion from the new army had left them without status or income, and from the global network of foreign fighters that Syria’s civil war attracted.

The June 2014 offensive that captured Mosul, Iraq’s second-largest city, with approximately 30,000 fighters defeating 60,000 Iraqi army soldiers who abandoned their weapons and fled, was the most stunning collapse of a Western-trained military force in the War on Terror’s history. The Iraqi army that the United States had spent approximately $25 billion to build and train dissolved because it had never developed the institutional loyalty, the non-commissioned officer culture, and the belief that its cause was worth fighting for that professional armies require. Its collapse revealed that the investment had built the appearance of military capability without the substance.

ISIS’s declaration of a caliphate and its requirement that all Muslims pledge allegiance or be subject to attack represented a theological and organisational innovation that distinguished it from Al-Qaeda. Where Al-Qaeda had remained a clandestine network conducting spectacular attacks, ISIS governed territory, provided services, collected taxes, and used the apparatus of a proto-state to demonstrate that Islamic governance was practically achievable in the present rather than merely aspired to in the future. This territorial and administrative dimension attracted a different kind of recruit than Al-Qaeda’s appeal to jihadist heroism, drawing people who wanted to live in what they believed was an authentic Islamic state rather than primarily those who wanted to conduct operations against the West.

The campaign to defeat ISIS territorially, which involved American air power, Kurdish Peshmerga and Syrian Democratic Forces on the ground, and eventually the reconstruction of Iraqi army capability, succeeded militarily by 2019. Mosul was retaken in July 2017 after nine months of fighting that destroyed much of the city. Raqqa, ISIS’s Syrian capital, fell in October 2017. Baghdadi was killed in an American special operations raid in October 2019. The territorial caliphate was eliminated, but the network dispersed rather than dissolved, continuing operations in Iraq, Syria, and increasingly in Africa and South Asia.

The Afghanistan Withdrawal

The American withdrawal from Afghanistan, completed on August 31, 2021 after twenty years of military presence, produced the most complete and most televised collapse of an American strategic objective in the War on Terror, as the Taliban reconquered the country in eleven days as American forces completed their withdrawal.

The Doha Agreement of February 2020, negotiated by the Trump administration and signed with the Taliban without the Afghan government’s participation, committed the United States to withdraw all forces by May 2021 in exchange for Taliban commitments not to allow Al-Qaeda to use Afghan territory and to engage in peace negotiations with the Afghan government. The Taliban met their commitment on the first condition sufficiently to prevent an immediate American withdrawal halt; they ignored the peace talks commitment entirely. The Biden administration extended the withdrawal deadline to August 31, 2021.

The Taliban’s offensive following the withdrawal’s announcement was so rapid that it overwhelmed not only American intelligence assessments but the assessments of virtually every observer. The Afghan army, which on paper had approximately 300,000 soldiers, effectively did not fight: commanders negotiated surrenders, soldiers changed sides or went home, and provinces fell in days. Kabul fell on August 15, two weeks before the withdrawal deadline, with President Ghani fleeing the country on a helicopter.

The chaos at Kabul’s Hamid Karzai International Airport in the final days of the withdrawal, as thousands of Afghans who had worked with American forces or had built their lives on the assumption of continuing American presence tried desperately to evacuate, was one of the most painful episodes in American foreign policy in decades. The suicide bombing by an ISIS-K operative on August 26 that killed thirteen American service members and approximately 170 Afghans near the airport’s Abbey Gate was the final American military casualty of the Afghan War.

The withdrawal’s most immediate consequence for the War on Terror was the question of whether Afghanistan would again become a sanctuary for Al-Qaeda operations against the United States. The Doha Agreement’s commitments were not consistently honoured: Al-Qaeda senior figure Ayman al-Zawahiri was killed by an American drone strike in Kabul in July 2022, his presence there demonstrating that the Taliban had not in fact denied Al-Qaeda access to Afghan territory as they had committed.

Key Figures

Dick Cheney

Dick Cheney’s role as Vice President from 2001 to 2009 was the most institutionally influential senior official in shaping the War on Terror’s architecture, and his specific influence on the decisions about Iraq, interrogation, and surveillance was disproportionate to his formal institutional position.

His conviction that September 11 had changed the risk calculus permanently, that the United States could not afford to wait for threats to fully materialise before acting against them, and that the executive branch required expanded authority to act on uncertain intelligence, drove the key decisions of the Bush administration’s first term. His advocacy for the Iraq invasion, his role in the legal framework for enhanced interrogation, and his management of the intelligence community’s assessment of Iraqi weapons of mass destruction all reflected this conviction.

His willingness to state publicly that the United States had engaged in waterboarding and that he would authorise it again, in interviews after his time in office, placed him in the extraordinary position of a former senior government official publicly confirming that his administration had used techniques that international law and American treaty commitments defined as torture. His defence of these decisions on consequentialist grounds, that they prevented attacks that would otherwise have occurred, was contested by the Senate Intelligence Committee’s findings but has remained his consistent position.

General David Petraeus

Petraeus was the War on Terror’s most effective military commander, whose development of counter-insurgency doctrine, command of the Surge in Iraq, and subsequent command in Afghanistan, represented the highest level of American military intellectual and operational performance of the period. His 2006 counter-insurgency field manual, co-authored with Marine General James Mattis, was the most important American military doctrinal document since the Cold War and demonstrated that the military had the capacity to learn from its failures.

His personal downfall, when he resigned as CIA Director in November 2012 following the revelation that he had shared classified information with his biographer and lover Paula Broadwell, was one of the War on Terror period’s most painful personal stories: the general who had done more than any other individual to redeem American military performance in Iraq, undone by a personal misjudgment that bore no relationship to his professional achievement.

Ahmed Chalabi

Chalabi, the Iraqi exile politician who convinced the Bush administration that post-Saddam Iraq would be a stable democracy grateful for American liberation, and whose fabricated intelligence about Saddam’s weapons and Al-Qaeda connections helped provide the justification for the invasion, represents the specific failure of American pre-war intelligence to distinguish between information that was accurate and information that told decision-makers what they wanted to hear.

His Defence Intelligence Agency funding, his provision of Iraqi exiles as intelligence sources whose claims were incorporated into intelligence assessments without adequate verification, and his specific influence on the case for war reflected a failure of the intelligence community’s analytical independence that post-war investigations documented thoroughly. His subsequent marginalisation in post-war Iraqi politics, as Iraqi voters proved less enthusiastic about him than his American backers had expected, completed the specific story of a policy built on wishful thinking.

Hamid Karzai

Karzai’s thirteen-year presidency of Afghanistan (2001-2014) combined genuine patriotism and genuine inability to govern the country into the sustained paradox of a leader whose authority the United States needed to legitimise and whose corruption and incompetence made that legitimacy impossible to build. His family’s involvement in the opium trade and in the patronage networks that the American aid and military contracting system had funded provided the Taliban with their most effective political argument: that the government in Kabul was a puppet of foreign interests whose officials enriched themselves while ordinary Afghans remained poor.

His complicated relationship with the United States, which ranged from cooperation to public criticism of American military tactics to eventual bitterness about the withdrawal, reflected both his genuine commitment to Afghan sovereignty and his inability to build the institutions that sovereignty required.

The Costs

The War on Terror’s human and financial costs represent the most direct measure of the policy’s consequences, and they require honest accounting before any assessment of what was achieved can be meaningful.

The approximately 929,000 deaths that Brown University’s Costs of War project estimated includes approximately 7,052 American military personnel, approximately 8,000 American contractors, approximately 177,000 opposition fighters in Afghanistan and Iraq, and approximately 300,000 to 400,000 civilians killed in the wars’ direct violence. The project estimates that indirect deaths from disease, displacement, and destruction of healthcare infrastructure may have killed several times as many civilians as the direct violence.

The approximately four to six trillion dollar cost estimate includes not only the direct appropriations for the wars and the counter-terrorism infrastructure but the long-term costs of veterans’ healthcare, the interest on the debt incurred to finance the wars, and the diplomatic costs of the relationships damaged by the Iraq invasion and the drone programme. The veterans’ healthcare costs alone, as the population of veterans who served in Iraq and Afghanistan ages and requires increasing medical and psychiatric support, are projected to be in the trillions of dollars over the coming decades.

The Taliban’s return to power in Afghanistan after twenty years of American effort and approximately two trillion dollars of expenditure is the most direct measure of the Afghan mission’s failure by its own stated objectives. Whatever American military and intelligence operations had accomplished in disrupting Al-Qaeda’s operational capability, the political objective of establishing a stable Afghan democracy that could sustain itself without American military support was not achieved.

The Legacy

The War on Terror’s legacy for American foreign policy and for the international order is still being defined, but several dimensions are already clear enough to assess.

The most significant institutional legacy is the security apparatus that September 11 created and that the War on Terror sustained. The intelligence community’s expansion, the surveillance systems built under the PATRIOT Act and executive authority, the special operations forces whose capabilities grew enormously during the wars, and the counter-terrorism cooperation mechanisms built with dozens of allies, all represent genuine capabilities that have disrupted operations and prevented attacks. Whether these capabilities are adequately constrained by democratic oversight, and whether the specific trade-offs between security and civil liberties that they reflect are appropriate, is an ongoing debate that the War on Terror’s history has not settled.

The most important strategic legacy is the demonstration that military power alone is insufficient to address the conditions that produce terrorism, and that the absence of a political strategy adequate to the military operations the United States conducts is a guarantee of strategic failure even when tactical military success is achieved. The Iraq War won its initial military campaign and lost its political objective; Afghanistan won its initial military campaign and lost its political objective. The pattern suggested a fundamental mismatch between American military capability and American political capacity for the sustained nation-building that transforming the conditions of terrorism required.

The War on Terror’s most important contribution to international relations may be the demonstration that it produced: that a liberal democracy under sufficient threat and fear pressure will abandon the values that define it in ways whose costs prove more durable than the emergency that produced them, and that rebuilding those values once they have been compromised is harder and slower than abandoning them was. The lessons history teaches from the War on Terror about the relationship between fear and democratic values, about the limits of military power in addressing political problems, and about the long-term costs of strategic miscalculation, are among the most important that the twenty-first century has so far produced.

Frequently Asked Questions

Q: What was the War on Terror and how long did it last?

The War on Terror was the global campaign of military operations, intelligence activities, law enforcement actions, and diplomatic efforts that the United States and its allies conducted following the September 11, 2001 attacks, directed against Al-Qaeda and subsequently against a broader range of jihadist organisations. The campaign was declared by President Bush on September 20, 2001 and has never formally ended: while the Biden administration announced in 2022 that it was ending the “global war on terror” as an organising framework, American counter-terrorism operations have continued in multiple countries. The term’s vagueness, describing war against a tactic rather than a country or an organisation, was both its political strength, enabling almost any military action to be justified as part of it, and its strategic weakness, providing no clear criteria for victory and no framework for knowing when it had been won.

Q: Why did the United States invade Iraq in 2003?

The official justifications for the 2003 Iraq invasion were that Saddam Hussein possessed weapons of mass destruction that posed a threat to American and allied security and that there was a connection between Iraq and Al-Qaeda. Both claims proved wrong: no weapons of mass destruction were found after the invasion, and no credible evidence of an operational Iraq-Al-Qaeda relationship was established. The actual motivations combined Cheney’s conviction that Saddam represented a threat that could not be left unaddressed, the neoconservative vision of democratic transformation in the Middle East, the institutional momentum that had been building since the 1991 Gulf War left Saddam in power, and the political environment of post-September 11 America in which arguing against military action was politically costly. The invasion’s consequences, including the rise of Al-Qaeda in Iraq and eventually ISIS, the sectarian civil war, and the roughly 500,000 Iraqi deaths, represent the War on Terror’s most consequential strategic failure.

Q: What happened in Afghanistan over twenty years?

The twenty-year Afghanistan War proceeded through several distinct phases. The initial 2001-2002 campaign rapidly overthrew the Taliban government and dispersed Al-Qaeda. The 2003-2007 period saw a deceptive stability in which the Taliban regrouped in Pakistan while American attention was concentrated on Iraq. The 2008-2014 period saw the Taliban insurgency intensify, multiple American troop surges, and the transition to Afghan security force leadership. The 2015-2021 period involved a continued American presence as an advisory and air support mission while the Taliban gradually expanded their territorial control. The August 2021 withdrawal produced the Taliban’s rapid reconquest of the country as the Afghan government and military collapsed. The twenty years of effort produced a Afghan army that did not fight, a democratic government that could not survive without American support, and approximately 3,500 coalition military deaths and approximately 70,000 Afghan military and police deaths, while failing to achieve the political objective of a self-sustaining Afghan democracy.

Q: What was the Islamic State and how did it emerge from the War on Terror?

The Islamic State (ISIS, ISIL, or Daesh) was a jihadist organisation that emerged from Al-Qaeda in Iraq, itself a consequence of the 2003 Iraq invasion. Under Abu Bakr al-Baghdadi’s leadership from 2010, it grew from the sectarian political environment created by Nouri al-Maliki’s sectarian Shia-dominated government, which had alienated and excluded Iraq’s Sunni population. In June 2014, ISIS captured Mosul and declared a caliphate, controlling approximately 34,000 square miles at its peak. Its distinctive characteristic was territorial governance: unlike Al-Qaeda, which remained a clandestine network, ISIS governed territory, collected taxes, administered justice, and provided services in the areas under its control. This territorial and administrative dimension attracted foreign fighters in unprecedented numbers, with approximately 40,000 from over 100 countries joining. The campaign to destroy ISIS’s territorial caliphate, conducted by American air power and local Kurdish and Iraqi ground forces, succeeded by 2019, but the network dispersed rather than dissolved.

Q: How effective was the drone programme?

The drone programme’s effectiveness is genuinely contested and depends on what criteria are applied and over what timeframe. By the criterion of killing identified Al-Qaeda and affiliated leaders, the programme was effective: it killed significant numbers of operational commanders and planners in Pakistan, Yemen, Somalia, and Syria who might otherwise have planned attacks that would have killed more people. The killing of Anwar al-Awlaki in 2011 removed AQAP’s most effective English-language propagandist and operational planner. By the criterion of reducing the overall terrorist threat, the programme’s effectiveness is less clear: the civilian casualties from drone strikes and the “signature strike” policy’s killing of people based on behavioural patterns rather than confirmed identities generated anti-American sentiment in affected communities that provided new recruitment conditions. The net counter-terrorism impact of trading killed operators for radicalised recruits has been estimated differently by analysts with different methodological approaches, and the honest answer is that the precise balance is unknown.

Q: What was the PATRIOT Act and how did it change American surveillance?

The USA PATRIOT Act, passed six weeks after September 11 with minimal Congressional debate, significantly expanded the federal government’s surveillance authorities. Its most significant provisions included Section 215, which authorised the collection of “any tangible things” relevant to terrorism investigations and was used to justify the bulk collection of telephone metadata revealed by Edward Snowden in 2013; the “sneak and peek” provisions that allowed searches without notifying the target; the expanded wiretapping authorities that reduced the threshold for judicial approval; and the material support provisions that made providing support to designated terrorist organisations a crime. The Act’s authorities were repeatedly reauthorised and in some cases made permanent; the Freedom Act of 2015 ended the bulk telephone metadata collection while preserving most other authorities. The PATRIOT Act’s passage reflected the specific political environment of the weeks after September 11, in which challenging any security measure was politically costly, producing legislation that extended executive authority well beyond what careful deliberation would have produced.

Q: What was the cost of the War on Terror?

The costs of the War on Terror were enormous by any measure. The Brown University Costs of War project estimated approximately 929,000 deaths directly attributable to the violence, including roughly 7,052 American military personnel and hundreds of thousands of civilians across Afghanistan, Iraq, Pakistan, Yemen, Syria, and other countries. Financial costs, including direct war appropriations, veterans’ healthcare, intelligence expansion, and interest on war-related debt, are estimated at between four and six trillion dollars, making the War on Terror the most expensive conflict in American history in nominal terms. The social costs, including the psychological wounds of the veterans who served, the communities destroyed by the violence in Afghanistan and Iraq, and the political costs of the democratic values compromised by the torture programme and mass surveillance, are harder to quantify but equally real. The strategic cost, measured by the distance between the stated objectives of creating stable democracies in Afghanistan and Iraq and the actual outcomes, is perhaps the most important cost of all.

Q: What did the War on Terror achieve?

The War on Terror achieved several significant objectives while failing at others. Its genuine achievements included the destruction of Al-Qaeda’s original Afghan operational infrastructure, which had trained the September 11 hijackers and dozens of other operatives; the killing or capture of much of Al-Qaeda’s senior leadership including Osama bin Laden; the disruption of numerous plots against Western targets through improved intelligence cooperation and counter-terrorism capabilities; and the significant reduction of Al-Qaeda’s ability to plan and execute large-scale attacks against the United States. Its failures included the Iraq War’s creation of conditions that produced Al-Qaeda in Iraq and eventually ISIS; the failure to achieve stable democratic governance in Afghanistan despite twenty years and approximately two trillion dollars of effort; the alienation of Muslim populations globally through civilian casualties, Abu Ghraib, Guantánamo, and the drone programme; and the damage to American democratic values and international reputation that the torture programme and mass surveillance produced.

Q: How did the War on Terror affect Pakistan?

Pakistan’s relationship to the War on Terror was the most complex and most consequential bilateral relationship in the entire campaign, combining official alliance with persistent double-dealing in ways that frustrated American strategy while reflecting Pakistan’s genuine security interests as Islamabad understood them. Pakistan provided over-flight rights, basing at Shamsi airfield for drone operations, and intelligence cooperation that contributed to the capture of multiple Al-Qaeda operatives. It also provided sanctuary to the Taliban leadership through ISI relationships, tolerated Al-Qaeda leadership in its major cities including Abbottabad where bin Laden was found, and actively supported the Haqqani Network and other Afghan Taliban factions as strategic assets against Indian influence in Afghanistan. The killing of bin Laden in Abbottabad without informing Pakistan, which embarrassed the Pakistani military establishment profoundly, was the most direct expression of the American assessment that Pakistani authorities at some level had known about bin Laden’s presence. The relationship’s fundamental dynamic, American dependence on Pakistani cooperation and Pakistani exploitation of that dependence to protect strategic assets, was never resolved and represents the War on Terror’s most important unresolved strategic problem.

Q: What was the War on Terror’s impact on civil liberties globally?

The War on Terror’s impact on civil liberties extended well beyond the United States to shape the legal and political environment in which governments globally claimed expanded counter-terrorism authorities. The British response, including the Terrorism Act 2000, the Anti-terrorism, Crime and Security Act 2001, and subsequent legislation that created control orders, extended detention without charge, and expanded surveillance authorities, reflected the same trade-off between security and civil liberties that the American PATRIOT Act represented, though within a different constitutional framework and with somewhat more sustained judicial oversight. European countries similarly expanded their counter-terrorism legal authorities, producing a broadly similar pattern of executive authority expansion that courts then had to constrain.

The global diffusion of counter-terrorism law included authoritarian governments that used the War on Terror’s framework to justify repression of political opposition, ethnic minorities, and religious groups as counter-terrorism measures. Russia’s Chechen operations, China’s repression of Uyghurs in Xinjiang, and various Middle Eastern governments’ expansion of security authorities were all framed in counter-terrorism terms that borrowed the War on Terror’s legitimating vocabulary. The broad claim that security required suspension of normal legal standards, which American and British officials had deployed in addressing genuine terrorist threats, proved easily adaptable to addressing political threats that were dressed in terrorist clothing.

Q: How has the War on Terror been evaluated in historical retrospect?

The historical evaluation of the War on Terror is still in progress, but the assessments of scholars, the participants themselves in their memoirs and testimony, and the investigations of multiple congressional committees and independent commissions have produced a broadly consistent account that acknowledges genuine achievements alongside profound strategic failures.

The intelligence community failures that allowed September 11 to succeed, documented by the 9/11 Commission, and the intelligence community failures that produced the Iraq WMD assessments, documented by the Senate Intelligence Committee and the Robb-Silberman Commission, represent the two most consequential analytical failures in American intelligence history. Their combined effect was first to fail to prevent the attacks and then to provide the flawed analysis that justified the most consequential strategic mistake of the response.

The military analysis is more mixed. American military forces demonstrated genuine competence in the initial phases of both Afghanistan and Iraq, and the Surge demonstrated the capacity for strategic adaptation even under unfavourable conditions. The fundamental problem was political rather than military: the inability to translate military gains into the political settlements that sustainable outcomes required, reflecting a civilian leadership that understood how to use military power but not how to achieve the political objectives that military power alone could not deliver.

The democratic values question, about whether the War on Terror’s departures from the legal and ethical standards that define liberal democracy were justified by the threat they addressed, has been answered differently by participants whose commitments differ. Those who believe that the enhanced interrogation programme prevented attacks that would have killed thousands of people assess the trade-off differently than those who believe the Senate Intelligence Committee’s finding that it produced little actionable intelligence that alternatives could not have generated. The persistence of Guantánamo, which no administration has been able to close despite executive orders directing closure, represents the specific difficulty of resolving the security-values tension that the War on Terror created in institutional form.

Q: What were the War on Terror’s effects on the broader Middle East?

The War on Terror’s effects on the broader Middle East extended well beyond Afghanistan and Iraq to reshape the region’s political landscape in ways that the campaign’s architects had not intended and in some cases had specifically sought to prevent. The removal of Saddam Hussein, who despite his crimes had been a secular Sunni counterweight to Iranian regional influence, allowed Iran’s expansion into Iraqi politics and through Iraqi territory toward Lebanon’s Hezbollah in ways that significantly extended Iranian regional reach. The destabilisation of Iraq and Syria created the conditions for ISIS’s emergence. The Arab Spring of 2011, which was not a direct product of the War on Terror but whose outcome was shaped by the regional instability it had contributed to, produced revolutions whose trajectory in Libya, Syria, Yemen, and Egypt was shaped partly by the armed groups, displaced populations, and institutional vacuums that the War on Terror’s regional interventions had created.

Saudi Arabia’s relationship to the War on Terror was simultaneously that of an essential American partner and a country whose citizens and whose religious and educational establishment had provided the ideological foundation for the jihadist movement. The specific tension between the strategic necessity of the Saudi relationship and the Saudi contribution to the conditions that produced terrorism was never resolved, and it represents one of the War on Terror’s most important unaddressed structural problems.

Tracing the arc from September 11 through Afghanistan, Iraq, the drone campaigns, and the ISIS emergence to the Afghanistan withdrawal that ended the formal American military presence in that country two decades after it began is to follow both the most important American foreign policy experience of the twenty-first century’s opening decades and one of the most instructive cautionary tales about the gap between military capability and political wisdom that modern history has produced.

Q: What was the role of Pakistan’s ISI in the War on Terror?

Pakistan’s Inter-Services Intelligence directorate (ISI) occupied the most contradictory position of any institutional actor in the War on Terror: it was simultaneously the United States’ most important regional intelligence partner and the primary institutional protector of the Taliban and the Haqqani Network, the two most dangerous armed groups that American forces were fighting in Afghanistan.

The ISI’s relationship with the Afghan Taliban dated from the 1990s, when Pakistan had sponsored the Taliban’s rise as a strategic tool for extending Pakistani influence in Afghanistan and as a buffer against Indian influence. The specific investment that Pakistan had made in the Taliban as a strategic asset was not something that the post-September 11 American alliance could simply demand to be abandoned, because abandoning it would leave Pakistan exposed to the Indian strategic encirclement that Pakistani strategists had always feared. The Americans could offer incentives and threaten consequences, but they could not alter Pakistan’s underlying strategic calculations.

The ISI’s continued maintenance of the Taliban leadership’s Quetta Shura (the Taliban’s governing council, based in Quetta, Pakistan) throughout the Afghan War was both documented by intelligence assessments and consistently denied by Pakistani officials. American military and intelligence officials who served in Afghanistan have described the specific absurdity of conducting operations against Taliban fighters who would return to Pakistan to regroup, rearm, and retrain under conditions that suggested institutional protection. The ISI’s contribution to the outcome of the Afghan War, by denying the Taliban the military defeat that would have been required to produce a negotiated settlement, was in some respects more consequential than any other single factor.

The Haqqani Network, which conducted some of the most sophisticated attacks against American and NATO forces including truck bombs and complex assaults on well-defended targets, maintained its leadership in Pakistan’s North Waziristan tribal area throughout the war. American pressure to act against the Haqqanis produced Pakistani military operations that were limited, half-hearted, and occasionally telegraphed the timing to the targets in advance. Admiral Mike Mullen, Chairman of the Joint Chiefs, described the Haqqani Network as a “veritable arm” of the ISI in 2011 congressional testimony, the most direct public statement by an American senior military official of what classified assessments had long concluded.

Q: What was the Special Inspector General for Afghanistan Reconstruction and what did it find?

The Special Inspector General for Afghanistan Reconstruction (SIGAR), established by Congress in 2008 to provide independent oversight of the approximately two trillion dollars spent on Afghanistan, produced the most comprehensive and most damning assessment of the Afghan reconstruction programme’s failures available to the public, documenting in quarterly reports the gap between official progress claims and ground-level reality.

SIGAR’s investigations documented a systematic pattern of projects that were completed on paper but non-functional in practice, security forces that existed in numbers on paper but were absent in the field, and governance programmes that measured inputs, the number of officials trained, the number of courts built, rather than outcomes, whether justice was actually being delivered. Its most striking contribution was “What We Need to Learn: Lessons from Twenty Years of Afghanistan Reconstruction,” published in 2021, which concluded that the United States had ignored evidence of what was failing and had prioritised the appearance of progress over honest assessment of conditions.

SIGAR’s chief finding was that the reconstruction programme had built structures, trained forces, and spent money without building the institutional capacity, the human capital, and the governance legitimacy that sustainable development requires. The Afghan army’s collapse in August 2021 was from SIGAR’s perspective entirely predictable: its quarterly reports had documented for years that the army’s reported strength overstated actual readiness, that attrition was high, that pay was often not delivered, and that unit cohesion and morale were insufficient to sustain combat without American support.

The political economy of reconstruction failure that SIGAR documented was partly a consequence of incentives: American contractors were paid to deliver projects, not to ensure they functioned; Afghan officials were incentivised to report success to maintain funding flows; and the institutional pressure within the military and civilian agencies to report progress to Congressional oversight produced optimistic assessments that did not reflect ground truth. The result was approximately two trillion dollars spent producing approximately twenty years of non-self-sustaining progress that evaporated in eleven days.

Q: What was the role of private military contractors in the War on Terror?

Private military contractors (PMCs) played an unprecedented role in both Afghanistan and Iraq, performing functions that ranged from guarding facilities and convoys to conducting interrogations and participating in combat operations, in ways that raised profound questions about accountability, democratic oversight, and the outsourcing of inherently governmental functions.

The contracting surge in Iraq was extraordinary in scale: at the peak of the occupation, there were more private contractors in Iraq than American military personnel. The companies included Blackwater (later Academi), which provided security services including the protection of senior American officials; MPRI and DynCorp, which trained Iraqi and Afghan security forces; KBR (formerly Halliburton’s Government and Infrastructure division), which provided logistics support including food service and base maintenance; and dozens of smaller companies performing specialised functions.

The Nisour Square massacre of September 16, 2007, in which Blackwater contractors killed seventeen Iraqi civilians in a Baghdad square in circumstances that Iraqi and American investigations both concluded were unjustified, was the most damaging single incident involving contractors and highlighted the accountability gap that private military contracting created. The contractors operated under no military law; their actions were excluded from Iraqi law by the Coalition Provisional Authority’s Order 17; and American civilian law’s application to contractors operating in foreign countries was legally uncertain. The eventual prosecution of Blackwater contractors years later demonstrated that accountability was possible but required sustained political effort and produced outcomes that many observers found inadequate to the scale of the harm.

The financial costs of contracting, which were substantially higher per unit of capability than using military personnel for comparable functions, reflected the rush to build up capability quickly without the time required to recruit and train additional military personnel. The contractors’ accountability to their shareholders and the incentive structures of defence contracting created systematic pressures toward cost overruns and performance gaps that military discipline at least partly constrained.

Q: How did the War on Terror affect the relationship between the executive and legislative branches?

The War on Terror produced the most significant expansion of executive branch authority since the Second World War, as the Bush administration claimed inherent constitutional authority to conduct surveillance, detain persons, and use military force in ways that it argued did not require Congressional authorisation, and as Congress proved consistently reluctant to constrain executive power in the security context.

The Administration’s “unitary executive” theory, which held that the President as Commander in Chief had inherent constitutional authority to conduct the War on Terror without the statutory limitations that Congress might impose, was applied to justify the torture programme, the surveillance expansion, and the indefinite detention of enemy combatants. The legal opinions that John Yoo and Jay Bybee wrote at the Office of Legal Counsel provided the legal scaffolding for these claims, and their subsequent repudiation by OLC’s own professionals did not occur until after the most significant decisions had already been made.

Congress’s response was a sustained pattern of deference that critics argued amounted to an abdication of its constitutional oversight responsibilities. The Authorization for Use of Military Force (AUMF) passed in September 2001, which authorised the President to use all necessary and appropriate force against those responsible for September 11, was interpreted by subsequent administrations to authorise operations against any group associated with Al-Qaeda in any country, an interpretation whose breadth went well beyond what the September 2001 Congress had contemplated but which Congress never formally challenged or revised. The 2002 AUMF for Iraq, passed on the basis of intelligence that proved inaccurate and that Congress did not adequately scrutinise, reflected the specific political difficulty of opposing military action in the post-September 11 environment.

The Snowden revelations of 2013 demonstrated that even the oversight mechanisms that Congress had created, including the Foreign Intelligence Surveillance Court and the Intelligence Committees, had not adequately constrained the surveillance expansion, partly because the oversight was conducted in classified settings that prevented public accountability, and partly because the Administration had not fully disclosed the scope of its activities even to the oversight bodies. The revelation that the NSA was collecting telephone metadata on virtually all Americans’ calls under a legal theory that the FISC had developed in secret, with no opportunity for adversarial challenge to the government’s interpretation, demonstrated the limits of secret oversight in constraining secret authority.

Q: What was the relationship between the War on Terror and the Arab Spring?

The Arab Spring of 2011, which produced mass uprisings against authoritarian governments across the Arab world, had a complex and contested relationship with the War on Terror, including both genuine ideological connections and important distinctions that the War on Terror’s democratic promotion framework sometimes obscured.

The connections were real but indirect. The War on Terror’s democracy promotion agenda, however inconsistently applied and however undermined by American support for authoritarian allies, had contributed to an international discourse about democratic governance in Arab societies that the Arab Spring’s participants engaged with. The information technology that the War on Terror’s period had spread globally, including social media platforms that enabled the coordination of mass protests, was the infrastructure through which the Arab Spring’s uprisings organised. And the humiliation of American military occupation in Iraq, and the democratic dysfunction that Iraq’s post-Saddam politics had demonstrated, paradoxically contributed to the conclusion among some Arab intellectuals that democracy required indigenous agency rather than foreign imposition.

The distinctions were equally important. The Arab Spring’s participants were not primarily motivated by jihadist ideology but by economic grievances, political humiliation, and demands for dignity and accountability. The specific movements that brought down Ben Ali in Tunisia and Mubarak in Egypt drew on secular liberal and Islamist elements in coalitions that differed fundamentally from the jihadist model. The peaceful character of the initial Tunisian and Egyptian uprisings, and their success in mobilising mass support without the violence that the War on Terror associated with political change in the Arab world, demonstrated possibilities that the jihadist framework had obscured.

The War on Terror’s institutional legacy complicated the Arab Spring’s outcomes in specific ways. In Libya, where NATO intervened under the R2P framework, the absence of the post-conflict planning that the War on Terror had demonstrated to be essential produced the chaos that followed Gaddafi’s fall. In Syria, where the Assad government’s violence was far more severe than anything the original uprisings had produced, the War on Terror’s history of regime change gone wrong inhibited intervention in ways that permitted the escalation of violence.

Q: What was the role of torture in the War on Terror and why was it wrong?

The torture programme, which the CIA conducted at black sites and which military personnel conducted at Abu Ghraib and other detention facilities, represents the War on Terror’s most straightforward moral failure, and understanding why it was both wrong and counterproductive is important for the specific reason that the circumstances that produced it, genuine fear of further attacks and genuine pressure on intelligence agencies to prevent them, are circumstances that could recur.

The legal case against the torture programme rests on multiple foundations. The United States had ratified the UN Convention Against Torture, which defines torture as any act by which severe physical or mental suffering is intentionally inflicted to obtain information or a confession, and explicitly states that no exceptional circumstances whatsoever, whether a state of war or a threat of war or any other public emergency, may be invoked as a justification of torture. The Justice Department’s Office of Legal Counsel opinions that defined torture so narrowly as to exclude the CIA’s techniques were subsequently withdrawn by the same office’s career professionals as legally inadequate. The Supreme Court’s Hamdan decision and the Detainee Treatment Act of 2005 ultimately established legal constraints that the Administration had previously argued did not exist.

The practical case against torture was documented by the Senate Intelligence Committee’s report, which found that the programme produced little or no intelligence that could not have been obtained through non-coercive means. The experienced FBI interrogators who had developed rapport-based interrogation techniques, and who objected to the CIA’s methods on both legal and effectiveness grounds, provided direct testimony that professional interrogation produced reliable intelligence that coercive methods, which produce unreliable information from subjects who will say anything to stop pain, did not. Khalid Sheikh Mohammed’s false confession while being waterboarded, which sent American intelligence agencies pursuing phantom plots, was one of the most documented examples of the specific intelligence problem that torture created.

The strategic case against torture was demonstrated by Abu Ghraib: the photographs became the most effective Al-Qaeda recruitment tool of the War on Terror period, radicalising young Muslims who might otherwise not have joined the jihadist movement and providing propaganda that directly undermined the democratic values argument that American foreign policy was supposed to embody.

Q: What lessons has the military drawn from the War on Terror?

The American military’s institutional learning from the War on Terror has been extensive but uneven, reflecting the genuinely difficult question of what the appropriate lessons are when a campaign that produced genuine military achievements also produced genuine strategic failures.

The counter-insurgency doctrine that Petraeus developed and codified in FM 3-24, the Army’s counter-insurgency field manual, represented the most direct institutional adaptation to the specific challenges of fighting insurgencies rather than conventional state militaries. Its emphasis on protecting the population rather than killing the enemy, on building legitimacy rather than simply demonstrating force, and on understanding the political dimensions of conflict, reflected lessons from Vietnam and from the early Iraq failures that the Army had not institutionalised after Vietnam and had to relearn under combat conditions.

The development of special operations forces capabilities was the other major military adaptation. The Joint Special Operations Command’s expansion, both in numbers and in authorities, produced the direct action capability that killed bin Laden and has been the primary instrument of counter-terrorism operations in multiple countries. The “find, fix, finish, exploit, analyse” targeting cycle that JSOC developed, integrating intelligence and military operations in a continuous loop, represented a genuine operational innovation that has been effective against specific leadership targets.

The debate about what the military should take from the War on Terror regarding large-scale nation-building operations reflects genuine disagreement about whether the failures were inherent in the mission or products of specific avoidable mistakes. Critics who argue that large-scale military nation-building is inherently beyond American military capability, and that the military should focus on conventional deterrence and limited special operations counter-terrorism, draw different lessons from the same evidence as those who argue that Afghanistan and Iraq could have succeeded with better civilian-military integration, more resources applied earlier, and more sustained political attention.

Q: How did the War on Terror affect veterans?

The War on Terror produced the largest cohort of combat veterans since Vietnam, with approximately 2.5 million Americans serving in Afghanistan and Iraq, and their reintegration into American society, care for their wounds both physical and psychological, and political voice in debates about the conflicts they fought in have all been defining dimensions of the War on Terror’s domestic legacy.

The scale of psychological wounds from the Afghanistan and Iraq wars was unprecedented in American military history. The combination of multiple deployments, the nature of counter-insurgency combat in which the enemy was often indistinguishable from the population and in which every roadside could conceal a bomb, and the homecoming experience of returning to a civilian society that often did not understand or engage with what veterans had experienced, produced rates of post-traumatic stress disorder and traumatic brain injury that the Veterans Administration struggled to address.

The suicide rate among veterans of the War on Terror period has been documented as substantially higher than among non-veterans of comparable age, reflecting the psychological wounds that military healthcare has been inadequately resourced to treat. The VA’s wait time scandals, in which veterans in need of mental health and other care were waiting months for appointments while the system struggled with demand, represented a specific failure of the political commitment to care for those who had been sent to fight.

The political voice of veterans has been complex. Some veterans became the most effective advocates for continued engagement, arguing that the sacrifices already made obligated continued commitment; others became the most effective advocates for withdrawal, arguing from direct experience that the missions as they had evolved were not achievable at sustainable cost. Both positions drew legitimacy from the authenticity of combat experience, and the political debate about the War on Terror was marked by the specific authority that veterans’ testimony carried.

Q: What did the War on Terror mean for America’s image in the world?

The War on Terror’s effect on America’s international image was one of its most consequential and most difficult-to-repair costs, producing the shift from the global sympathy of September 11’s immediate aftermath to the global criticism that the Iraq War, Abu Ghraib, Guantánamo, and the drone programme generated over the following decade.

The Pew Global Attitudes surveys, which tracked international views of the United States through the War on Terror period, documented the decline. The United States had enjoyed historically high levels of positive sentiment globally in the weeks after September 11; by 2007, following the Iraq War and the Abu Ghraib photographs, positive views had declined sharply in most countries including traditional allies. The damage was particularly severe in the Muslim world, where the combination of two major invasions, the torture revelations, and the Palestinian conflict produced assessments of American foreign policy that were overwhelmingly negative.

The Obama administration’s initial international reception, including the Nobel Peace Prize awarded in anticipation of a changed American approach, reflected the degree to which the Bush administration’s War on Terror conduct had damaged American standing and the degree to which its reversal was expected to restore it. The continuation of drone strikes, the maintenance of Guantánamo despite executive orders directing closure, and the Libya and Syria interventions demonstrated that the change was more limited than initial expectations had suggested, producing a partial recovery in American standing that fell short of the pre-war baseline.

The specific damage to American credibility on human rights and democratic governance, which had previously given American diplomacy the moral authority to challenge other governments’ conduct, proved more durable than the policy changes. Countries that had been criticised for detention without trial or surveillance could point to Guantánamo and the NSA programmes as evidence that American human rights advocacy was selective and self-interested, undermining the standing from which American diplomacy could press others on governance issues.

Q: What is the current state of the War on Terror globally?

The global counter-terrorism landscape in the mid-2010s shows both the achievements of the decade and a half of counter-terrorism effort and the diffusion of jihadist networks that the effort’s failures have contributed to.

Al-Qaeda’s original organisation has been substantially degraded. The senior leadership that planned September 11 is almost entirely dead or imprisoned: Osama bin Laden was killed in 2011, Ayman al-Zawahiri was killed by drone strike in 2022, and most of the operational leadership that KSM had built has been killed or captured. The organisation that remains is a shadow of what it was in 2001, capable of inspiring attacks but far less capable of directing them with the operational detail that September 11 represented.

The diffusion of jihadist ideology has proceeded faster than the degradation of specific organisations. Al-Qaeda affiliates operate across West Africa, the Sahel, East Africa, Yemen, and parts of South Asia. ISIS, while territorially defeated in Iraq and Syria, has affiliates in Afghanistan, Libya, Egypt’s Sinai, parts of sub-Saharan Africa, and has inspired attacks in Europe, North America, and Australia. The “lone wolf” attack model, in which individuals radicalised online without direct organisational contact conduct attacks using accessible weapons, has proven difficult to disrupt through the intelligence methods that work against organised networks.

The War on Terror’s institutional framework, the legal authorities, the surveillance capabilities, the special operations forces, and the counter-terrorism cooperation mechanisms, remains largely in place. Whether this infrastructure is appropriately calibrated to the current threat, which is more diffuse and less centrally organised than the September 11-era Al-Qaeda, and whether the civil liberties costs of maintaining this infrastructure are justified by its security benefits, are questions that democratic societies continue to debate without reaching settled answers. The Cold War produced institutions and frameworks that persisted for decades after the threat they were designed to address had diminished; the War on Terror’s institutional legacy seems likely to follow a similar trajectory, whatever the specific future of jihadist terrorism as a threat.

Q: What was the relationship between the War on Terror and the Syrian Civil War?

The Syrian civil war that began in March 2011 and that had killed approximately 500,000 people and displaced approximately 13 million by 2016 was the War on Terror’s most consequential unresolved crisis, combining the regional effects of the Iraq War with the specific dynamics of the Arab Spring in a conflict that has drawn in virtually every regional and global power.

The Iraq War’s regional consequences shaped the Syrian conflict’s character in important ways. The flow of weapons, fighters, and jihadist ideology from Iraq into Syria created the specific population of experienced jihadist fighters that the Syrian conflict attracted. The Alawite-Sunni sectarian dynamic that defined the Assad government’s relationship to the Sunni majority opposition resonated with the Shia-Sunni dynamics that the Iraq War had intensified regionally, drawing Iran and Hezbollah to Assad’s support and Gulf states to the opposition’s support in a proxy war that reflected the broader Saudi-Iranian rivalry.

ISIS’s emergence in Syria was directly facilitated by the Syrian civil war, which provided the ungoverned territory in which it built its territorial caliphate. The Obama administration’s response, which combined rhetorical opposition to Assad (“Assad must go”) with inadequate military support for moderate opposition and eventually with direct military operations against ISIS rather than Assad, reflected the specific difficulty of a policy that opposed both the Syrian government and its jihadist opponents without having the means to achieve either objective. The specific failure of the programme to train and equip Syrian opposition fighters, which produced approximately 54 trained fighters before being largely abandoned at a cost of approximately $500 million, was one of the War on Terror period’s most embarrassing programme failures.

The conflict’s humanitarian consequences, including the largest refugee crisis since the Second World War as Syrians fled to Turkey, Lebanon, Jordan, and eventually to Europe, produced the political pressures in European countries that contributed to the nationalist political movements whose electoral effects reshaped European politics. The connection between the War on Terror’s regional consequences and European domestic politics, through the specific pathway of the Syrian refugee crisis and the security concerns that attended it, is one of the War on Terror’s most indirect but most politically consequential effects.

Q: How did the War on Terror change military technology and tactics?

The War on Terror produced the most significant evolution in American military technology and tactics since the Cold War, driven by the demand for capabilities that the counter-terrorism and counter-insurgency missions required and that conventional Cold War force structures had not developed.

The drone programme’s technological evolution was the most publicly visible dimension. The Predator drone, which had been used in limited reconnaissance missions before September 11, was adapted for strike missions and operated as the primary counter-terrorism tool in Pakistan, Yemen, and Somalia from approximately 2004 onward. The Reaper drone, with larger payload and longer range, extended the programme’s reach. The integration of drone surveillance with intelligence analysis and direct action strike, the “find, fix, finish” cycle, produced a pace of operations against terrorist leadership that no previous counter-terrorism tool had achieved.

The counter-IED (improvised explosive device) effort drove the development of Mine-Resistant Ambush Protected (MRAP) vehicles, which replaced the Humvees that proved fatally vulnerable to the roadside bombs that were the insurgency’s primary weapon, and of electronic jamming technologies that disrupted the remote detonation systems that improvised explosive devices used. The investment in counter-IED technology, which amounted to approximately $75 billion, produced vehicles and jamming systems that became standard equipment for forces operating in IED environments.

Biometric collection and analysis, the systematic collection of fingerprints, iris scans, and facial recognition data from the Iraqi and Afghan populations, created the capability to identify insurgents across multiple encounters and to track individuals through their biometric signatures. The biometric database built in Afghanistan and Iraq was one of the War on Terror’s least discussed technological achievements, with implications for surveillance capability that extended well beyond counter-terrorism applications.

Special operations forces’ technological advancement, including the satellite communications, night-vision equipment, small arms improvements, and intelligence integration that made JSOC’s operations possible, produced a military capability significantly more effective than what existed before September 11. The raid that killed bin Laden demonstrated the specific combination of intelligence, special operations capability, and technological integration that had been developed over a decade of War on Terror operations.

Q: What was the impact of the War on Terror on American democracy?

The War on Terror’s impact on American democracy was profound and multidimensional, touching the relationship between the branches of government, the civil liberties framework, the democratic culture of debate and dissent, and the specific mechanisms through which democratic accountability is supposed to constrain executive power.

The most direct institutional impact was the executive branch’s expansion of authority, which the post-September 11 security environment enabled and which legislative deference and judicial reluctance to engage with national security matters allowed to accumulate. The President’s surveillance programme, the torture authorisations, the indefinite detention authorities, and the military commissions system were all constructed through executive branch legal opinions rather than through legislation, creating an executive branch claim to inherent authority that the Constitution’s text and prior practice did not clearly support.

The democratic culture of debate was specifically distorted by the fear environment that September 11 created and that administration officials periodically reinforced. The political cost of opposing security measures in the post-September 11 environment was demonstrated by the career consequences suffered by officials who raised concerns about the Iraq intelligence, by journalists who published information about classified programmes, and by the public labelling of war critics as insufficiently committed to American security. The specific episodic reinforcement of fear through elevated threat levels, which critics argued correlated with political events rather than actual intelligence assessments, maintained the emotional environment in which democratic debate was suppressed.

The longer-term democratic impact of the War on Terror is visible in the polarisation of American politics that it contributed to, the erosion of trust in government institutions that both the Iraq WMD failure and the Snowden surveillance revelations produced, and the specific form of nationalism that the War on Terror period’s security culture enabled. Whether American democracy has the institutional resilience to absorb these strains without permanent damage is a question that the War on Terror’s history poses without answering, and that the democratic institutions themselves must answer through their performance.

Q: What is the War on Terror’s relationship to refugee and migration crises?

The War on Terror’s connection to the refugee and migration crises of the mid-2010s is one of its most far-reaching and most politically consequential indirect effects, running through the Afghanistan and Iraq Wars to the Syrian civil war to the European political disruptions that Syrian and other refugee flows produced.

The Afghanistan and Iraq Wars generated substantial refugee populations: approximately 2.5 million Afghans were displaced by the conflict at various points, and approximately four million Iraqis were displaced by the sectarian violence of 2006-2007. These populations strained neighbouring countries’ capacity, particularly Jordan and Lebanon in Iraq’s case and Pakistan and Iran in Afghanistan’s, and some portion eventually reached Europe, contributing to the migration pressures that produced political reactions across the continent.

The Syrian civil war’s refugee crisis was the most dramatically politically consequential, as the approximately 1.3 million Syrians who sought asylum in Europe in 2015-2016 produced the political pressures that transformed European politics. The refugee flows, combined with the terrorism attacks in Paris (November 2015), Brussels (March 2016), and other European cities by ISIS-inspired or ISIS-directed attackers, created the specific political environment in which anti-immigrant and nationalist parties achieved their most significant electoral successes since the Second World War, including contributions to the Brexit vote in Britain and the rise of nationalist parties in France, Germany, Italy, and elsewhere.

The connection between the War on Terror’s regional consequences and these European political developments is indirect but real: the Iraq War’s destabilisation contributed to the Syrian conflict’s conditions; the Syrian conflict produced the refugee flows; the refugee flows produced the political pressures; and the political pressures produced the nationalist reactions that reshaped European politics. The casualty chain runs from specific decisions made in Washington in 2002-2003 to political outcomes in European democracies a decade and more later.

Q: What would a successful War on Terror have looked like?

The counterfactual question of what a successful War on Terror would have looked like is the most intellectually clarifying lens through which to assess what actually happened, because it requires specifying what success would have meant and then assessing how the choices made did or did not serve those objectives.

Success in the specific counter-terrorism mission, disrupting Al-Qaeda’s operational capability and preventing large-scale attacks against the United States, was substantially achieved through the combination of intelligence improvements, special operations, drone strikes, and the capture or killing of much of Al-Qaeda’s senior leadership. This achievement was real and significant, and it was accomplished without the large-scale follow-on attacks that intelligence assessments had predicted in the months after September 11 were imminent. A more successful counter-terrorism programme would likely have pursued these objectives with less militarised means, more emphasis on intelligence and law enforcement cooperation, and fewer of the civilian casualties that the drone programme generated.

Success in the political objectives, creating self-sustaining democratic governance in Afghanistan and Iraq that would address the conditions producing terrorism, proved unachievable with the resources and attention applied and possibly unachievable at any sustainable cost. A more successful approach to the political objectives would likely have involved longer time horizons, more graduated expectations, more genuine engagement with the populations being governed rather than with their official representatives, and the acceptance that institutional development in societies with limited prior democratic experience requires generations rather than years.

Success in maintaining American values, conducting the War on Terror in ways that preserved the legal and ethical standards that define liberal democracy, was entirely within the administration’s capacity but was not achieved. The torture programme, Guantánamo, and the surveillance expansion were choices that reflected specific institutional pressures and specific official decisions rather than genuine security necessity, and a more successful War on Terror would have pursued the security objectives while maintaining the legal standards, as the FBI’s interrogation success and the justice system’s record of prosecuting terrorism cases demonstrated was possible.

The most important lesson from this counterfactual exercise is that the War on Terror’s failures were not primarily military or intelligence failures but political and values failures: failures of strategic judgment, failures of democratic oversight, and failures to maintain the principles that the stated objective of defending American values required. These are failures that future democratic governments facing comparable threats can choose to avoid, if they are willing to learn from the record that the War on Terror has produced.

Q: How did the War on Terror affect alliances and multilateral institutions?

The War on Terror’s impact on American alliances and multilateral institutions was one of its most consequential long-term diplomatic effects, straining relationships that had been built over decades and that provided the framework of cooperation that American foreign policy required.

NATO’s invocation of Article 5 following September 11 was the clearest possible demonstration of allied solidarity, and the subsequent International Security Assistance Force in Afghanistan drew on that solidarity to build a coalition of over fifty countries that contributed forces at various points. This allied contribution was both militarily significant and politically important, providing the multilateral legitimacy that American leadership required. The sustained allied commitment to Afghanistan, even as the mission evolved beyond its original counter-terrorism purpose into the nation-building enterprise that most allied governments had not signed up for, demonstrated the depth of alliance relationships when the United States maintained genuine consultation and genuine respect for allies’ perspectives.

The Iraq War shattered this framework. France and Germany’s opposition to the invasion, and their threatened Security Council veto, produced the “with us or against us” response that administration officials applied to allied disagreement. The dismissal of French and German concerns as evidence of timidity or anti-Americanism, rather than as legitimate strategic analysis from allies with different perspectives and different risk assessments, reflected the specific hubris of an administration that believed its own assessment was not subject to allied challenge. The bilateral damage to French-American and German-American relations took years to repair.

The UN Security Council’s role, which the administration had circumvented by proceeding with the Iraq invasion without authorisation, was damaged in ways that constrained subsequent multilateral operations. The precedent of a major power proceeding with military action over Security Council objection, while not unprecedented in Council history, reinforced the specific Russian and Chinese conclusion that American humanitarian intervention claims were pretexts for regime change that they needed to veto systematically, contributing to the Security Council deadlock on Syria that prevented multilateral action in the world’s worst humanitarian crisis of the decade.

Q: What is the significance of the Abbottabad raid for the War on Terror?

The Abbottabad raid of May 2, 2011, in which Navy SEAL Team Six killed Osama bin Laden at his compound in a Pakistani military city, was the War on Terror’s most symbolically and strategically important single military operation, and its circumstances revealed as much about the War on Terror’s conduct as the operation itself achieved.

The raid’s success was the product of approximately nine months of intelligence work following a lead about bin Laden’s courier network, and represented the culmination of the decade-long effort to locate and kill the man most directly responsible for September 11. The operational planning, conducted in total secrecy from Pakistani authorities, and the execution, in which two stealth helicopters flew approximately 150 miles from Jalalabad to Abbottabad, demonstrated the specific military capability that the post-September 11 investment in special operations forces had produced.

The decision not to inform Pakistani authorities before or during the raid, and the revelation that bin Laden had been living for years in a compound in Abbottabad, a city that houses Pakistan’s Military Academy and is home to numerous retired military and ISI officers, produced the most acute crisis in the American-Pakistani relationship of the entire War on Terror period. The Pakistani government’s public outrage at the violation of its sovereignty masked, incompletely, the private embarrassment that either Pakistani officials had known of bin Laden’s presence and concealed it, or had not known and had demonstrated that the state apparatus in a major city was unable to detect the world’s most wanted man living openly nearby. Neither possibility was comfortable, and the American assessment that the former was more likely has shaped American-Pakistani relations since.

Bin Laden’s killing did not end Al-Qaeda or the War on Terror, as the subsequent emergence of ISIS and the persistence of Al-Qaeda affiliates demonstrated. But it removed the symbolic and operational leader whose continued freedom had represented both a practical counter-terrorism failure and a political failure to deliver the justice that September 11 had demanded. Obama’s announcement of the killing was one of the genuinely cathartic moments of the War on Terror’s two-decade history, even as the strategic context made clear that it was not an ending but a significant milestone in a continuing campaign.

Q: How did September 11 and the War on Terror reshape American national identity?

The War on Terror’s effect on American national identity was one of its least discussed but most consequential dimensions, as the attacks and the response to them reinforced and in some cases transformed specific strands of American self-understanding that had been developing before September 11 and that the decade and a half of conflict amplified.

The military and security culture’s elevation to central prominence in American public life was perhaps the most visible change. The valorisation of military service, the “support our troops” culture that made questioning the wars politically difficult to distinguish from questioning the soldiers, and the expansion of military parades and displays at sporting events all reflected a public relationship with the military that differed from the ambivalent post-Vietnam relationship that had preceded September 11. The genuine respect that the soldiers, sailors, and Marines who served in genuinely difficult and genuinely dangerous conditions deserved was overlaid with a political culture that used the military’s moral authority to insulate the political decisions about deployment from scrutiny.

The relationship between American national identity and American Muslim identity was permanently altered. The September 11 attacks associated Islamic identity with terrorism in ways that affected not only policy but daily life for Muslim Americans, whose names, appearance, and religious practice became triggers for the specific surveillance, profiling, and social discrimination that the post-September 11 period institutionalised and that legal challenges have only partially addressed.

The American exceptionalism that had informed the democracy promotion agenda and that had led the neoconservatives to believe that American military power could transform Middle Eastern societies, was tested by the Afghanistan and Iraq Wars’ outcomes. The gap between the expectations and the results produced a chastened reassessment of American power’s limits that was itself a form of national identity adjustment, as the country that had believed it could reshape the world in its image confronted the evidence that the world was more resistant to reshaping than the optimism of the unipolar moment had suggested.

The lessons history teaches from the War on Terror about the relationship between national fear and national values, about the costs of confusing military capability with political wisdom, and about the gap between the ideals that democracies claim to embody and the policies they implement under pressure, are among the most important that the twenty-first century has so far produced, and they remain directly relevant to the choices that democratic societies face whenever the combination of threat, fear, and concentrated power that the War on Terror produced is recreated by whatever the next crisis brings.